This invention relates to an electrographic recording medium of the type having a double-layer coating consisting of a dielectric layer and a conductive layer on the surface of a support for use in facsimile or high speed printing by way of example.
One type of known electrographic recording media comprise a conductive layer which is formed on a surface of a support such as a paper sheet or a plastic film and has a surface resistivity of 10.sup.5 -10.sup.11 ohms and a dielectric layer coated on the conductive layer and made of a highly dielectric material whose resistivity (specific resistance) is above 10.sup.12 .OMEGA.cm.
Formerly, the conductive layer was formed usually by impregnating a support such as of slick paper with a solution of an inorganc electrolyte material such as lithium chloride or by coating a surface of a support with either a cationic polyelectrolyte such as a high molecular quaternary ammonium salt or an anionic polyelectrolyte such as a high molecular sulfonate. However, a conductive layer of this category, i.e. one utilizing ionic conduction in an electrolyte, has a serious drawback that the surface resistivity of this layer is greatly influenced by the humidity in the environmental atmosphere and, particularly, undergoes a drastic increase where the himidity in terms of relative humidity is below about 20%, so that recording becomes almost impossible in very low humidity atmospheres. The reason for such a drastic increase in the surface resistivity in a very low humidity atmosphere is that the conductive layer is deprived of moisture indispensable to ionic conduction.
To obviate this drawback of the conductive layer utilizing ionic conduction in an electrolyte, it has been proposed to utilize a metal iodide such as cuprous iodide or silver iodide, which is an electronically conductive substance, as an essential material of the conductive layer in an electrographic recording medium of the aforementioned type in, for example, U.S. Pat. No. 3,245,833 and Japanese Patent Application Publication (primary) Nos. 48(1973)-30936 and 50(1975)-159339. Since the surface resistivity of an electronically conductive layer of the proposed type is not significantly influenced by the humidity in the environmental atmosphere, recording becomes possible even at very low humidities. However, it is inevitable that the use of either cuprous iodide or silver iodide gives an unwantedly colored recording medium. Besides, such an iodide is thermally unstable since its electronic conductivity originates in the excess of iodine, so that a recording medium utilizing either cuprous iodide or silver iodide tends to liberate iodine, an ill-smelling vapor, during thermal fixing of toner-developed images.